Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to suspension systems for supporting a component of a vehicle in a manner that isolates that component from vibrations in other sections of the vehicle, such as isolating the operator cab or seat from vibration of the chassis as the vehicle travels over rough terrain; and more particularly to active hydraulic suspension systems in which an actuator is drive to counter the vibration.
2. Description of the Related Art
Vibration has an adverse affect on the productivity of work vehicles in which an operator cab is supported on a chassis. Such vehicles include agricultural tractors, construction equipment, and over the road trucks. The vibrations experienced by such vehicles reduce their reliability, increase mechanical fatigue of components, and most importantly produce human fatigue due to motion of the operator""s body. Therefore, it is desirable to minimize vibration of the vehicle cab or seat where the operator sits and of other components of the vehicle.
The operator of off-highway vehicles is subjected to large amplitude, low-frequency vibration when traveling over rough terrain. Previous vehicle cab suspension systems often performed poorly in the range of vibration frequencies to which the human body is most sensitive, i.e. one to ten Hertz. When subjected to vertical movement, or bounce, the human abdomen resonates at approximately four to eight Hertz and the head and eyes resonate at ten Hertz. The upper torso resonates in response to pitch and roll motion at between one and two Hertz. As a consequence, it is desirable to isolate the cab or seat of the vehicle from these vibration frequencies to improve the operator""s comfort and increase equipment productivity.
Traditional approaches to vibration isolation employed either a passive or an active suspension system to isolate the vehicle cab or seat along one or more axes to reduce bounce, pitch, and roll of the vehicle. Passive systems typically placed a series of struts between the vehicle chassis and the components to be isolated. Each strut included a parallel arrangement of a spring and a shock absorber to dampen movement. This resulted in good vibration isolation at higher frequencies produced by bumps, potholes and the like. However, performance a lower frequencies, such as encountered by a farm tractor while plowing a field, was relatively poor. The lower frequency vibrations can be in the same range as the natural frequency of the system, thereby actually amplifying the vibration. One approach to decrease that amplification has been to increase the damping ratio xcex6 given by the expression   ζ  =      C          2      ⁢                        m          ⁢                      xe2x80x83                    ⁢          K                    
where C is the damping coefficient, m is the mass being isolated, and K is the spring rate. Unfortunately increasing the damping ratio compromises isolation of vibration at the higher frequencies.
In addition, a two-point passive isolation system, designed to reduce roll vibration (left and right movement) creates an undesirable inertial roll of the cab when the vehicle turns. Generally, if the isolated mass, e.g. the cab, is allowed to roll with a passive suspension, a torsion bar must be added to provide stiffness which resists the inertial roll produced by a vehicle turn. The addition of a torsion bar not only adds cost to the system, it also reduces the effectiveness of other suspension components.
Active suspensions place a cylinder and piston arrangement between the chassis and the cab or seat of the vehicle to isolate that latter component. The piston divides the cylinder into two internal chambers and an electronic circuit operates valves which control the flow of hydraulic fluid to and from each chamber.
U.S. Pat. No. 4,887,699 discloses one type of active vibration damper in which the valve is adjusted to control the flow of fluid from one cylinder chamber into the other chamber. The valve is operated in response to one or more motion sensors, so that the fluid flow is proportionally controlled in response to the motion.
U.S. Pat. No. 3,701,499 describes a type of active isolation system in which a servo valve selectively controls the flow of pressurized hydraulic fluid from a source to one of the cylinder chambers, and drains oil from the other chamber back through a return line to the source. A displacement sensor and an accelerometer are connected to the mass which is to be isolated from the vibration and provide input signals to a control circuit. In response, the control circuit operates the servo valve to determine into which cylinder chamber fluid is supplied, from which cylinder chamber fluid is drained and the rate of those respective flows. This control of the cylinder produces movement of the piston which counters the instantaneous vibration motion.
Although an active isolation system is particularly effective in the low vibration frequency range, there are cost and performance penalties to use this type of system for higher frequency vibration. Because the system has to respond more rapidly at the higher frequencies, the measurement of mass movement and the reaction of the servo valve often introduce a phase lag in the counter motion, unless relatively expensive, very fast-acting components are employed. In addition, fluid must be supplied to the active suspension from the vehicle""s hydraulic system. During prolonged vibrating conditions, such as when an agricultural tractor is plowing a field, the constant draw of the fluid from the tractor""s pump requires that the hydraulic system have increased capabilities.
An active suspension system for isolating a first body from a second body has at least one hydraulic actuator connected between the first body and the second body. Each hydraulic actuator comprises a cylinder and a piston received within the cylinder thereby dividing the cylinder into a first chamber and a second chamber. The first and second chambers are coupled to a hydraulic circuit node, and an accumulator also is connected to the node. An electrically operable valve selectively connects the node to a source of pressurized hydraulic fluid or to a reservoir. A sensor detects movement of the first body and produces an electrical signal indicating the detected movement. A controller responds to the electrical signal from the sensor by operating the valve to move the piston relative to the cylinder so as to attenuate transmission of movement of the second body with respect to the first body.
In a first embodiment of the active suspension system, the node is directly connected to the first chamber and to the second chamber. In a second embodiment, a check valve permits fluid to flow only in a direction from the first node to the second node, and a fixed orifice is connected in parallel with the check valve. In a third version of the active suspension system, the fixed orifice is replaced with a variable orifice controlled by the controller.
The sensor may detect an amount of displacement between of the first body and the second body. Alternatively, the sensor detects inertial motion of the first body. In a preferred embodiment of the active suspension system, both types of sensors provide input signals to the controller.